Such pressure sensors with hydraulic pressure transfer involve, as a rule, a measuring apparatus having two half-chambers, which are, in each case, sealed by a separating membrane, or diaphragm, and filled with a transfer medium. The separating membranes are loaded, respectively, with a pressure to be measured and with a reference pressure, and these pressures are then transferred via the separating membranes into the respective half-chambers. The half-chambers are separated from one another by a sensor element, which has a pressure sensitive element, especially a measuring membrane, which is loaded on its first surface with the hydraulic pressure in the first half-cell and on its second surface with the hydraulic pressure in the second half-cell.
The separating membranes have finite stiffnesses, so the pressure in each half cell differs by a pressure dPm from the pressure on the separating membrane, with the pressure deviation dPm being a function of the deflection of the separating membrane.
Since the measuring element of the sensor element is usually very stiff, the deflection of the separating membrane due to fluctuations of the external pressure can, to a first approximation, be neglected. The significant deflections are caused by the thermal expansion of the transfer medium in the half-chambers of the measuring apparatus.
Due to boundary-conditions of the design and due to manufacturing tolerances, it is very difficult, or practically impossible, to construct a measuring mechanism having perfectly symmetrical half-chambers. That is, small deviations remain between the volumes of the half-chambers, and the stiffnesses of the two separating membranes are not absolutely identical. This has the consequence that, for example, in the case of a warming of the transfer liquid in the two half-chambers, the measuring element is loaded from its two, pressure sensitive sides, with a different hydrostatic pressure, although, for example, externally, identical pressures are acting on the two separating membranes. This leads to a temperature-dependent shifting of the zero point of the measurement signal.